In recent years, NMR spectroscopy has become one of the most important analytical tools in organic chemistry, as well as in structural biology and biochemistry. However, compared to other spectroscopic techniques, NMR spectroscopy is a relatively insensitive method, requiring samples concentrations in the micromolar and even millimolar range. Nevertheless, NMR provides an enormous amount of details about the chemical organization and the structure of compounds. The introduction of new equipment and techniques has helped to improve the sensitivity in NMR experiments. One important improvement has come in the form of cryogenic probes, which generate and receive the NMR pulses being used during examination of a sample. The sensitivity increase is achieved by cooling the radio frequency (RF) receiver coils to temperatures of 15 to 30° K. The colder coils have lower resistance and, therefore, higher quality factors (Q). This, in turn, increases the signal amplitude and lowers the thermal noise. Both the higher Q-factor and the lower noise result in an increase in the overall signal-to-noise (S/N) ratio and therefore in the sensitivity of the apparatus.
The sensitivity of cryogenic probes, however, is also dependent on the use of samples and sample solutions having the appropriate electrical characteristics. An electrically conductive sample, such as one using a conventional buffer typical of protein structure determinations, will add a resistance to the coil, which can significantly reduce the signal-to-noise ratio. However, many biological macromolecules must be studied in buffered solutions to keep the pH constant and the molecule in a defined protonation state. Moreover, in many cases, salts are added to a buffer to increase solubility and to prevent aggregation of the investigated biomolecules. But salt concentrations of 100 to 150 mM, which are typical for many biological samples, significantly decrease the sensitivity advantage of a cryogenic probe.